There are known in the prior art, as disclosed in U.S. Pat. No. 3,674,838, unsaturated carbamoyloxy carboxylates having the formula ##STR1## wherein R.sup.1 is hydrogen, a monovalent alkyl, aryl or aralkyl hydrocarbon radical free of ethylenic unsaturation of one to eight carbon atoms, or a halogen; R.sup.2 is a divalent hydrocarbon radical of one to 12 carbon atoms; and R.sup.3 is hydrogen or a lower alkyl radical of one to eight carbon atoms.
These unsaturated compounds are useful as polymerizable monomers, particularly in coating compositions, wherein they serve as viscosity reducers. They impart excellent adhesion and flexibility retention properties to the cured coatings in which they are incorporated.
A number of methods are known for the preparation of these compounds. These methods include (a) the reaction of an hydroxyacrylate, such as 2-hydroxyethyl acrylate, and an isocyanate of the formula R.sup.3 --N.dbd.C.dbd.O; (b) the reaction of an hydroxyacrylate, such as 2-hydroxyethyl acrylate, with phosgene to produce a chloroformate which is subsequently reacted with ammonia to form the carbamoyloxy acrylate; (c) the reaction of an hydroxyacrylate, such as 2-hydroxyethyl acrylate, with urea; (d) the reaction of an hydroxy carbamate, such as 2-hydroxyethyl carbamate, with acryloyl chloride; and (e) the transesterification reaction between an acrylic ester and an hydroxy carbamate, such as 2-hydroxyethyl carbamate.
There are various disadvantages associated with all of the above methods of producing carbamoyloxy acrylates. For example, methods (a), (b) and (d) give high yields of the desired products, but require the use of highly toxic and/or expensive materials. Method (c) employs reaction temperatures on the order of 130.degree. C. to achieve commercially acceptable reaction rates, but at this temperature polymerization of the acrylic moiety is difficult to prevent. We have found that method (e), using typical transesterification catalysts (e.g., tetraisopropyl titanates), sometimes produces a commercially unacceptable yield of the desired product, and these products are highly colored.
Direct esterification--reaction of acrylic or methacrylic acid with an appropriate carbamate alcohol--has generally not been employed to produce the compounds of the above formula because of the well known difficulties in esterifying higher alcohols with acrylic or methacrylic acid. The reaction conditions which tend to promote the esterification reaction also tend to promote polymerization of the acrylyl moieties. The prior art teaches preferred temperatures for esterification of "higher" alcohols (i.e., greater than 3 carbon atoms) of 95.degree. C. and higher, such as disclosed in Canadian Pat. No. 768,651. However, the acrylate esters which are disclosed as being preparable by that process are recoverable as distillates, whereas the unsaturated carbamoyloxy carboxylates can only be conveniently recovered as residue products. This causes a problem in that the polymerization inhibitors which are effective at inhibiting polymerization of the acrylyl moiety at temperatures above 95.degree. C. (e.g., phenothiazine, methylene blue, and hydroquinone) impart color to the product, which color is difficult or impossible to remove by means other than distillation of the product. Thus, carbamoyloxy carboxylates containing acrylyl moieties by conventional esterification processes would not be suitable for use in applications where it is desired to have a low-color product.